Eustatic sea level changes cause alternating periods of subaerial exposure, flooding, or progradation at the shelf-slope boundary. Eustatic falls of sea level that are more rapid than subsidence cause subaerial exposure and canyon cutting. Rises of eustatic sea level coupled with subsidence commonly cause flooding of the shelf margin, marine transgression, and sediment starvation of the middle and outer shelf. Stillstands or slow falls of eustatic sea level that are less than the rate of subsidence commonly cause marine regressions where, in many places, the shoreline progrades out to the shelf-slope boundary.

The effect of eustatic sea level changes on the shelf-slope boundary is readily observable on seismic data despite changes in the rate of subsidence and rate and type of deposition. Seismic and well data from offshore northwest Africa are used to demonstrate these relations. In this area, high subsidence rates followed the opening of the Atlantic in the Early Jurassic and gradually changed to very slow subsidence rates in the Tertiary. Depositional rates increased throughout the Jurassic, reaching a maximum in the Early Cretaceous. Rates of deposition were very low in the Late Cretaceous and Early Tertiary and increased again in the late Tertiary. Sediment type changed from primarily carbonate in the Jurassic to sands, silts, and shales in the Cretaceous and Tertiary.

This and other examples demonstrate that sea level change is the major factor affecting the shelf-slope boundary in different tectonic settings. The only known cause for the high rates of sea level change determined in these studies is glaciation. Such studies indicate major phases of glaciation occurred periodically throughout the Phanerozoic. Timing of possible glacial periods is shown for the Jurassic and Tertiary.

End_of_Article - Last_Page 1003------------